Hollow fibers and thin walled hollow tubes are commonly used in mass transfer, heat exchange, and cross flow particle filtration devices. In these applications the hollow tubes or fibers provide a high surface to volume ratio which permits a greater transfer of heat and mass in a smaller volume than a device made with flat sheet materials of similar composition.
A hollow fiber or a hollow tube comprises an outer diameter and surface, an inner diameter and surface, and a porous or non-porous material between the first and second surfaces or sides of the tube or fiber. The inner diameter defines the hollow portion of the fiber or tube and is used to carry one of the fluids. For what is termed tube side contacting, a first fluid phase flows through the hollow portion, sometimes called the lumen, and is maintained separate from a second fluid phase, which surrounds the tube or fiber. In shell side contacting, the first fluid phase surrounds the outer diameter and surface of the tube or fibers and the second fluid phase flows through the lumen. In an exchange apparatus, packing density relates to the number of useful hollow fiber or hollow tubes that can be potted in the apparatus.
Examples of applications in semiconductor manufacturing where heating of a liquid is required include sulfuric acid and hydrogen peroxide photoresist strip solutions, hot phosphoric acid for silicon nitride and aluminum metal etching, ammonium hydroxide and hydrogen peroxide SC1 cleaning solutions, hydrochloric acid and hydrogen peroxide SC2 cleaning solutions, hot deionized water rinses, and heated organic amine based photoresist strippers.
Cooling of heated liquids after use in a bath, especially photoresist stripping solutions, phosphoric acid, SC1 and SC2 cleaning solutions is necessary prior to disposing of the used chemical. Electrochemical plating baths and apparatus are sometimes maintained at sub-ambient temperatures.
On a wafer processing track apparatus, accurate and repeatable conditioning of the temperature of liquids such as spin on dielectrics, photoresists, antireflective coatings, and developers prior to dispense onto a wafer requires heating or cooling of these liquids.
Heat exchangers are devices which transfer heat from one fluid, the process fluid, and a second working fluid. Polymer based heat exchangers are used for heating and cooling chemicals for these applications due to their chemical inertness and resistance to corrosion. However, polymeric heat exchange devices are usually large because a high heat transfer surface area is required to effect a given temperature change due to the low thermal conductivity of the polymers used in the device. Braiding of the tubes is used prevent the tubes from becoming unevenly spaced when used in open container heat exchange applications. Such devices take up valuable space, requires large holdup volumes of chemicals or exchange fluid, and are costly to make. Such devices also require o-ring seals which are prone to failure and are also a source of ionic and particulate contamination.
Quartz heaters are also used to heat liquids used for semiconductor processing. Quartz is susceptible to breakage and exposed resistively heated surfaces pose fire and explosion hazards especially for organic liquids and liquids which evolve flammable gases.
Gas to liquid contactors or exchangers using hollow fiber tubes are used in semiconductor manufacturing to remove or to add gases to liquids. Commercially available gas to liquid contactors utilize baffles to improve mass transfer between the fluids. Typical applications for contacting membrane systems are to remove dissolved gases from liquids, “degassing”, or to add a gaseous substance to a liquid. For example, a wet bench is a wafer processing apparatus where ozone gas is added to very pure water to be contacted with semiconductor wafers for cleaning and oxide growth.
Cross flow filtration is used in semiconductor manufacturing to remove suspended solids, such as abrasive particles used in chemical mechanical polishing slurries. A chemical mechanical slurry stream contains in addition to the solid slurry material, oxidizers like hydrogen peroxide in combination with acids and bases such as hydrochloric acid or ammonium hydroxide. A chemical mechanical polishing tool is an example of a wafer processing apparatus used in semiconductor manufacturing.
To effect cross flow filtration, mass transfer, or heat transfer using contactors made from hollow tubes or porous hollow fibers, baffling is commonly used to promote flow across the tubular elements. Various designs for baffling have been detailed in the literature which improve the transfer of heat and materials to the hollow tubes. U.S. Pat. No. 5,352,361 teaches the art of baffling for hollow fiber gas to liquid contactors. Such baffles are useful for polyethylene like hollow tubes where methods to pot and spin laminate baffles are easily implemented. Baffling of perfluronated tubes is not practical using this technique. U.S. Pat. No. 4,749,031 teaches baffling with perfluorinated baffles through which individual hollow tubes are threaded. It is cumbersome, and expensive to manufacture exchange contactors using this technique. U.S. Pat. No. 4,360,059 describes a spiral heat exchanger prepared from a cast material such as aluminum. Such a method does not contemplate the use of thermoplastics nor does it address the need for the substantially higher surface area required for low thermally conductive thermoplastic materials.
U.S. Pat. No. 3,315,740 discloses a method of bonding tubes together by fusion for use in heat exchangers. Tubes of a thermoplastic material are gathered in a manner such that the end portions of the tubes are in a contacting parallel relationship. The end portion of the gathered tubes is placed within a sleeve having a thermoplastic internal surface and being rigid relative to the tubes. A fluid heated to a temperature at least equal to the softening point of the thermoplastic material is introduced into the interiors of the end portions of the tubes. Then a pressure differential is imposed across the walls of the tubes so that the pressure within the tubes is greater than the pressure on the exterior surfaces of the tubes, thereby causing the tubes to be expanded and to be fused with the surfaces of the adjacent tubes. Such a method produces an irregular pattern of entrances to the hollows of the tubes effecting non-uniform flow distribution to the tubes. Such a method also requires relatively thick walled tubing to provide sufficient thermoplastic to form a seal with the housing sleeve. It is not contemplated to use such a potting method to form an end structure or a unified terminal end block, nor is it contemplated to braid the tubes and thermally set them prior to potting to provide enhanced flow distribution.
Canadian Patent 1252082 teaches the art of making spiral wound polymeric heat exchangers. Such a device requires mechanical fixtures to hold the tubes in place and as such requires a large volume of space.
U.S. Pat. Nos. 4,980,060 and 5,066,379 describe fusion bonded potting of porous hollow fiber tubes for filtration. The invention does not disclose the conditions required to effect fusion bonding of non-porous thermoplastic tubes for preparation of a unified terminal end block for use in phase and heat exchange. The invention does not contemplate twisting or braiding of the hollow fibers nor does it contemplate annealing the fibers prior to potting to effect a structure on the potted tubes for enhanced flow distribution.
Alan Gabelman and Sun-Tak Hwang in the Journal of Membrane Science, volume 159, pp 61-106, 1999 describe the importance of uniform fiber spacing for obtaining better mass transfer in hollow fiber contactors. The authors observe that hand built modules have more uniform fiber spacing but that the cost of such modules do not justify their higher manufacturing cost. Such arguments can be applied to hollow tube heat exchange and cross flow devices as well.
U.S. Pat. No. 5,224,522 describes a method and device for producing woven hollow fiber tapes for use in exchange devices such as blood oxygenators and heat exchangers. Such a device method requires expensive and complicated weaving equipment to fix the fibers in a preferred relationship in the tube mats.
Currently it is impractical to use thermoplastic heat exchangers for large heat loads, shell side liquid flow, or efficient shell side cross flow filtration because of the high expense and large size of devices needed. Metal heat exchangers are unacceptable for use in semiconductor manufacturing because of the corrosive nature of the chemicals and also because of the need to eliminate metallic and particulate impurities from process liquids. What is needed is a thermoplastic apparatus for heat exchange, mass transfer, or cross flow filtration with high surface area, uniform fiber spacing, and minimal volume. The apparatus should eliminate the need for baffles.